Persons with mobility impairments often depend on a wheelchair or walking aid to facilitate mobility. As a result, they are frequently subjected to physical barriers and obstacles such as stairs and curbs. The ADA legislation requires that these physical barriers be removed. Ramps provide some access; however, ramps can be very long and difficult to climb. Further, depending on the elevation change and available space, ramps may be impractical. One solution is a wheelchair lift. Wheelchair lifts for commercial buildings and private residences must be designed and tested to meet the requirements of the ASME Code: A18.1, SAFETY STANDARD FOR PLATFORM LIFTS AND STAIRWAY CHAIRLIFTS.
Low-rise platform lifts, or lifts that are limited to 24-inch maximum vertical travel, have been developed for use in courtrooms, church pulpits, meeting chamber podiums, and other similar environments. These types of installations not only provide a means for safe level changes, but must also be sensitive to decorum and surrounding architecture.
As low-rise lifts are being incorporated into new and remodel construction, obstacles are being encountered that require alternative lifting mechanisms to facilitate a simpler and cleaner interface with surrounding millwork finishes. For example, screw column type lifting mechanisms require the screw columns to be encased within the millwork walls, which directly influences and sometimes restricts the placement of screw columns and requires significant modifications to existing decorative finishes.
One suitable lifting assembly is a “scissor-type” lifting mechanism. Such lifting mechanisms typically have very high lifting ratios at the lower range of platform travel, and very low lifting ratios at the upper range of platform travel. This ratio differential causes the platform speed to vary throughout the range of vertical travel, which typically must be overcome with the use of hydraulics. Hydraulic systems are undesirable as they are known to bleed or leak over time.
Scissor mechanisms also tend to have a profile larger than desirable, as space is required within the scissor envelope for the actuator (or actuators). This space problem is often overcome with the use of a pit under the lift to house the actuator. However, retrofit applications often do not have sufficient pit space available. As a result, scissor lifts are undesirable in applications that require a pit to house actuator of the scissor lift.
Thus, a low-profile lift mechanism, that is discrete, achieves a suitable lifting power, and maintains a relatively constant lifting speed is desired.